EMC Design of RF PCB (3)
1. Wiring
1.1 Impedance Control
The impedance of PCB signal traces is related to the dielectric constant of the board, PCB structure, line width, etc. Generally, the RF signal traces should be routed on the surface layer as much as possible. In some cases, the inner layer can be routed. The most common is the third layer to route the stripline, and the impedance is 50 ohms. The following table lists the parameters controlled by the impedance (50) of a typical PCB previously designed, and these data can be directly applied to the newly designed PCB:
2.2 Corner
If the RF signal traces run at right angles, the effective line width at the corner will increase, and the impedance will decrease, causing reflections. Therefore, to handle corners, there are two corner methods:
Chamfered and rounded corners.
The chamfer is suitable for relatively small curved chamfers as shown in the figure. The applicable frequency of chamfering is up to 10GHz.
The radius of the arc corner should be large enough, generally speaking, to ensure: R>3W. as the picture shows.
1.3 Microstrip wiring
The RF signal is carried on the top layer of the PCB, and the plane layer below the RF signal must be a complete ground plane to form a microstrip line structure. As shown in Figure 9. To ensure the structural integrity of the microstrip line, it must be done:
(1) The edges on both sides of the microstrip line must be at least 3W wide from the edge of the ground plane.
(2), and within the range of 3w, there must be no non-grounded vias.
(3) It is forbidden for RF signal traces to cross the ground plane gap on the second layer.
(4) The edge of the microstrip line extends to both sides along the electric field, and a ground copper sheet should be added between the uncoupled microstrip lines, and a ground via hole should be added on the ground copper sheet. (5) The distance from the microstrip line to the shielding wall should be kept above 2W. (W: line width)
1.4 Microstrip line coupler
It is often used to detect the strength and standing wave of high-power signals. If the requirements are not high and the coupling degree is greater than 20dB, two close PCB traces can be used to make a microstrip coupler, as shown in Figure 10(a). When directionality is required, the coupling length L is:
L=in/4
W is the width of the coupling line, and generally the impedance of the microstrip line must be guaranteed to be 50.
1.5 microstrip power divider
In the case of low requirements, PCB traces can be used to make a microstrip line power divider. As shown in Figure 10(b). It is necessary to ensure that the impedance meets the following requirements:
Z = 502
Z = 212 Z = 70.7
The wiring distance LBc from the power combining point B to the resistance point C should meet the following formula: LBc = input/4 The
resistance value of the resistor is 1002.
1.6 Basic components of microstrip line
(1), Microstrip line segment (a) The equivalent circuit element can be expressed as (d) jwL = jZosin 0jwC = j(1/Zo)tg(0/2) The characteristic impedance Zo of the
microstrip line is relatively high, and the microstrip line segment It has the effect of series inductance; the characteristic impedance of wide microstrip line is low, which is equivalent to parallel capacitance.
(2) The equivalent circuit element of the microstrip parallel open-circuit branch (b) is (e)Zop = -j(1/Zo)ctg0op when the length of the branch line is 0. ,<90°, that is, when the mechanical length is less than g/4, it is equivalent to inductive reactance.
(3) The equivalent circuit element of microstrip parallel short-circuit branch © is (f)
Zsh = j(1/Zo)tg0sh
When the length of the branch line is 0.<90°, the parallel short-circuit branch is equivalent to the parallel inductive reactance: 0. When >90°, it is equivalent to capacitive reactance. With these three microstrip components, various microstrip circuits can be formed with various changes. These microstrip circuits have a certain filtering effect. The most widely used microstrip component is the /4 microstrip line, and an application example is mentioned below.
Two points at a quarter wavelength (90°) apart of a periodic sine wave have minimal influence on each other.
When one end of the input /4 microstrip line is directly grounded, or grounded through a high-frequency filter capacitor (such as 100pF), that is, when one end is AC grounded, the other end is equivalent to an AC open circuit, which has an inductive effect on a signal whose line length is equal to 2/4 , its typical application is the bias and power supply circuit of small signal amplifier tube or power amplifier tube, as shown in the figure.
The key points of PCB design are as follows:
(1). The length of the bias trace at the output end of the power amplifier tube is I/4, which is the distance from the nearest high-frequency filter capacitor to the signal trace or the matching copper skin.
(2) The length of the offset trace at the input end of the power amplifier tube is λ/4, which is the distance from the nearest high-frequency filter capacitor to the signal trace or the matching copper skin.
(3) The combined filter capacitors connected in parallel should be arranged together, and the order of arrangement should be paid attention to, as shown in Figure 12. The high-impedance wire that enters /4 should be pulled out directly from the pin of the high-frequency filter capacitor.
1.7 Stripline Routing
Some radio frequency signals must pass through the middle layer of the PCB, the most common is to go from the third layer, the second layer and the fourth layer must be a complete ground plane, that is, the eccentric stripline structure. as the picture shows. To ensure the structural integrity of the stripline. Must do:
(1) The edges on both sides of the stripline must be at least 3W wide from the edge of the ground plane.
(2), and within the range of 3W, there must be no non-grounded vias.
(3) It is forbidden for RF signal routing to cross the ground plane gap of the second or fourth layer.
1.8 Both sides of the RF signal traces are covered with ground copper
It is required that the distance between the ground copper skin and the signal traces is ≥1.5W, and ground wire holes are added to the edge of the ground copper skin.
The edge of the copper skin of the ground wire should be smooth and flat, and no sharp burrs are allowed; except for special purposes, it is forbidden to protrude excess wire ends from the RF signal routing.
2. Other Design Considerations
Add "RF" characters on the silk screen for PCB processing and finished board inspection, according to the special requirements of RF PCB.
Due to the high operating frequency of RF devices, the internal input of the device cannot be directly protected by a protection circuit, which is more susceptible to electrostatic breakdown than other devices. Therefore, when designing the PCB, an eye-catching anti-static mark should be added on the silk screen.